Introduction to ggplot2

ggplot2

The grammar of graphics: consistent aesthetics, multidimensional conditioning, and step-by-step plot building.

• dataset with mappings from variables to aesthetics
• one or more layers
• one scale for each aesthetic mapping
• a coordinate system,
• the facet specification (conditioning)

library(ggplot2)

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p <- ggplot(mpg, aes(x=hwy, y=cty))
p + geom_point()

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Simple scatterplot

p <- ggplot(mtcars, aes(x=wt, y=mpg))
p + geom_point()

Add aesthetic mappings

Set color by # of cylinders

p + geom_point(aes(colour = factor(cyl)))

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Set shape using # of cylinders

p + geom_point(aes(shape = factor(cyl)))

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Adjust size by qsec

p + geom_point(aes(size = qsec))

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Add a linear model

p + geom_point() + geom_smooth(method="lm")

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Change scale color

p + geom_point(aes(colour = cyl)) + scale_colour_gradient(low = "blue")

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Change scale shapes

p + geom_point(aes(shape = factor(cyl))) + scale_shape(solid = FALSE)

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Set aesthetics to fixed value

ggplot(mtcars, aes(wt, mpg)) + geom_point(colour = "red", size = 3)

Transparancy

Varying alpha is useful for large datasets

d <- ggplot(diamonds, aes(carat, price))
d + geom_point(alpha = 1/10)

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d + geom_point(alpha = 1/20)

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d + geom_point(alpha = 1/100)

Other Plot types

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Three Variables

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Stats

Visualize a data transformation

• Each stat creates additional variables with a common ..name.. syntax
• Often two ways: stat_bin(geom="bar") OR geom_bar(stat="bin")

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Specifying Scales

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Discrete color: default

b=ggplot(mpg,aes(fl))+geom_bar( aes(fill = fl)); b

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Discrete color: brewer

b+scale_fill_brewer( palette = "Blues")

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Discrete color: greys

b+scale_fill_grey( start = 0.2, end = 0.8, na.value = "red")

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Continuous color: defaults

a <- ggplot(mpg, aes(hwy)) + geom_dotplot( aes(fill = ..x..)); a

## stat_bindot: binwidth defaulted to range/30. Use 'binwidth = x' to adjust this.

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Continuous color: gradient

a + scale_fill_gradient( low = "red", high = "yellow")

## stat_bindot: binwidth defaulted to range/30. Use 'binwidth = x' to adjust this.

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Continuous color: gradient2

a + scale_fill_gradient2(low = "red", high = "blue", mid = "white", midpoint = 25)

## stat_bindot: binwidth defaulted to range/30. Use 'binwidth = x' to adjust this.

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Continuous color: gradientn

a + scale_fill_gradientn( colours = rainbow(10))

## stat_bindot: binwidth defaulted to range/30. Use 'binwidth = x' to adjust this.

Coordinate Systems

Position

Themes

Colophon

Sources: * ggplot cheatsheet *

Licensing: * Presentation: CC-BY-3.0 * Source code: MIT